Method for Bending Metal Sheet to Achieve High Angle Accuracy

ABSTRACT

A method for controlling the bending of a metal sheet includes the steps of: establishing a relational database on a relationship among material parameters, forming angles, amounts of springback, and amounts of press of an upper die; blanking and reverse-engineering a material parameter of a metal sheet to be processed; comparing the material parameters in the relational database with the material parameter of the metal sheet to be processed which is obtained by the reverse-engineering; and performing an bending operation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal sheet processing technology, and specifically to a method for increasing accuracy of bending a metal sheet.

2. Description of the Related Art

Bending is an important method for processing metal sheets in the industrial production. In a process of producing metal sheet members, the angle of the bent metal sheet increases due to the elastic recovery of the metal material. In addition, there are large variations in performance among raw materials with different specifications and from different makers, making the amounts of springback of the materials after bending differ. This makes the accuracy of forming structural members unstable. For this reason, an increase in the forming accuracy by effectively avoiding the springback is very important in the forming process.

Against this background, CN203140585U discloses a method for increasing the accuracy of forming stamped parts by designing a new V-shaped bending die for the parts to reduce the springback. However, a great restriction is imposed on such method. The thus-designed bending die can be used for a bending process requiring only one forming angle, and the products formed using the bending die is in one size only. The same die cannot be used when the forming angle of products is changed, or when the size of products is increased. Thus, a set of dies for the corresponding angle and size needs to be newly processed. This results in a very large production cycle very and very high costs.

CN201020102617 discloses a V-shaped 90° self-supporting die capable of adjusting an amount of bending springback and compensating for a springback angle. The die includes a punch and a lower die. The punch includes an upper die base. Angle-adjusting side plates capable of adjusting a stamping angle of a lower end of the punch are attached to two ends of the upper die base. The angle-adjusting side plates are shaft-coupled in a bending radius section. The lower die includes a lower die base. Left and right self-supporting plates are connected to an upper end of the lower die base with a hinge.

According to the patent disclosed in CN201020102617, the compensation is achieved by adjusting the formed angle. The compensation also needs to be carried out by: performing bending tests while repeating an angle adjustment process; testing the forming angle; thereby predicting the amount of springback; and eventually adjusting the die angle. This method requires so many manual operations, and cannot be implemented with an automated control. Furthermore, dies used for such method are complicated, and after used for long time, the dies are deformed. Moreover, larger dies need to be designed for larger bending materials. However, the rigidity of the thus-designed larger dies cannot be secured, and the dies maybe warped or deformed during the forming processes, adversely affecting the forming quality.

With these taken into consideration, the present invention aims at obtaining a relational database on a relationship among amounts of springback, forming angles, material performances and sheet thicknesses of metal sheets by simulating bending processes using a CAE (Computer-Aided Engineering) simulation technique, based on a principle that the amount of bending springback is different among materials depending on properties of the materials, without directly controlling the amounts of springback. Thus, for each material, the amount of springback can be known before the material is actually bent. Furthermore, for each material, accuracy requirements can be satisfied by: compensating for the springback in advance; and forming a bent member at one time. The forming angle can be automatically controlled. It is no longer necessary to adjust a grinder by manually measuring the forming angle. Both the automation level and the forming accuracy are high. This method can satisfy forming demands for different material performances, different sheet thicknesses, and different forming angles, as well as can process products in accordance with accuracy requirements.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems and makes it an object thereof to provide a method for controlling the bending of a metal sheet while capable of accurately controlling an angle at which to bend the metal sheet.

The present invention provides a method for controlling bending of a metal sheet comprising the steps of:

(1) establishing a relational database on a relationship among material parameters, forming angles, amounts of springback, and amounts of press of an upper die,

the establishing including

-   -   establishing a finite element simulation model for forming based         on a structure parameter of a die used for a bending process,         obtaining a simulated amount of springback corresponding to a         forming angle by inputting a material parameter of a metal sheet         for a simulation into the finite element simulation model for         the forming and thus simulating the bending process of the metal         sheet for the simulation, and using the simulated amount of         springback as an amount of springback to be added to the         relational database, and     -   after measuring material parameters of different metal sheets,         simulating the bending process of each different metal sheet         based on the obtained finite element simulation model for the         forming, and for each of the different metal sheets,         establishing a relational database on a relationship among a         material parameter, a forming angle, an amount of springback,         and an amount of press of the upper die;

(2) blanking and reverse-engineering a material parameter of a metal sheet to be processed,

the blanking and reverse-engineering including

-   -   blanking the metal sheet to be processed, and thereby obtaining         a blank, and     -   obtaining an experimental load-stroke curve of the metal sheet         to be processed during a blanking experiment by measurement,         establishing a finite element simulation model using a method         for reverse-engineering an actual physical parameter of the         metal sheet, simulating the blanking, and repeatedly         reverse-engineering the material parameter of the metal sheet to         be processed,

(3) comparing the material parameters in the relational database with the material parameter of the metal sheet to be processed which is obtained by the reverse-engineering,

-   -   if a reference material parameter, between which and the         material parameter of the metal sheet to be processed an error         is less than a second threshold value, exists in the relational         database, performing a bending operation, and     -   if a reference material parameter, between which and the         material parameter of the metal sheet to be processed an error         is less than a second threshold value, does not exist in the         relational database, like in the establishing the relational         database, simulating the bending process using the material         parameter of the metal sheet to be processed which is obtained         by the reverse-engineering, adding a relationship which is         obtained by the simulation among the material parameter of the         metal sheet to be processed, a forming angle, an amount of         springback, and an amount of press of the upper die to the         relational database, making the material parameter of the metal         sheet to be processed which is obtained by the         reverse-engineering become the reference material parameter of         the metal sheet to be processed in the relational database, and         subsequently performing the bending operation; and

(4) performing the bending operation,

the performing including

-   -   inputting a bending angle, and     -   calculating a needed amount of press of the upper die based on a         formula representing a relation between the forming angle         corresponding to the reference material parameter and the amount         of press of the upper die of the die of a bending machine for         performing the bending operation, sending the needed amount of         press of the upper die to the bending machine, and controlling         the bending machine in a way that the bending machine bends the         blank.

With the use of the foregoing method, the present invention obtains the relational database on a relationship among amounts of springback, forming angles, material performances and sheet thicknesses of metal sheets by simulating bending processes using a CAE simulation technique, based on a principle that when bend, materials show different amounts of springback depending on properties of the materials, without directly controlling the amounts of springback. Thus, for each material, the amount of springback can be known before the material is actually bent. Furthermore, for each material, accuracy requirements can be satisfied by: compensate for the springback in advance; and forming a bent member at one time. The forming angle can be automatically controlled. It is no longer necessary to adjust a grinder by manually measuring the forming angle. Both the automation level and the forming accuracy are high. This method can satisfy forming demands for different material performances, different sheet thicknesses, and different forming angles, as well as can process products in accordance with accuracy requirements.

The present invention provides the method for controlling bending of a metal sheet, in which the structure parameter of the die used for the bending process may include a draw radius of the upper die and an opening of a lower die in the die.

The present invention can establish the finite element simulation model for forming using the structure parameters of the die, which include the draw radius of the upper die and the opening of the lower die. Using this small number of parameters, the present invention can obtain the finite element simulation model for accurately simulating actual forming conditions. Thus, the present invention increases the efficiency of establishing the finite element simulation model.

The present invention provides the method for controlling bending of a metal sheet, in which in order to establish the finite element simulation model for the forming, an actual amount of springback of the metal sheet for the simulation may be obtained by performing a bending test, and a difference between the actual amount of springback and the simulated amount of springback may be calculated, and

if the difference is greater than a first threshold value, the simulation may be repeatedly performed by refining a grid size of the finite element simulation model for the forming until the difference becomes less than the first threshold value.

With the use of the foregoing method, the present invention can simulate the bending process using the CAE simulation technique with 98 percent accuracy, and can makes the predicted value almost equal to the actual value. Thereby, the present invention ensures that the accuracy requirements are satisfied by forming the bent member at one time. The present invention can automatically control the forming angle. The present invention makes it no longer necessary to adjust the grinder by manually measuring the forming angle. The present invention improves the automation level and the forming accuracy.

The present invention provides the method for controlling bending of a metal sheet, in which each material parameter used to establish the relational database on the relationship among the material parameters, the forming angles, the amounts of springback, and the amounts of press of an upper die may include a performance parameter and a flow stress curve of a material measured by a tensile test for detecting the material.

Since the present invention performs the finite element simulation on the forming process using the flow stress curve as the material parameter, the present invention can achieve higher simulation accuracy and efficiency.

The present invention provides the method for controlling bending of a metal sheet, in which the first threshold value may be 2%.

The present invention provides the method for controlling bending of a metal sheet, in which the bending process may be simulated, and a relationship between a different bending angle θ′ and an amount ΔH of press may be recorded, subsequently, an amount Δθ of springback corresponding to the different bending angle θ′ may be predicted using the finite element simulation model, and a relationship between the different bending angle θ′ and the amount Δθ of springback may be obtained, and based on θ=θ′+Δθ representing a relationship among a forming angle θ, the bending angle θ′ and the amount Δθ of springback, a relationship between the forming angle θ and the amount ΔH of press may be obtained

The present invention provides the method for controlling bending of a metal sheet, in which the second threshold value may be 1%.

The present invention provides the method for controlling bending of a metal sheet, in which if the reference material parameter, between which and the material parameter of the metal sheet to be processed an error is less than the second threshold value, does not exist in the relational database, the bending process may be simulated by inputting the material parameter of the metal sheet to be processed, which is obtained by the reverse-engineering, into the finite element simulation model, thereby, a relationship between a forming angle θ of the metal sheet to be processed and an amount ΔH of press may be obtained, and the relationship between the forming angle θ of the metal sheet to be processed and the amount ΔH of press may be sent to the relational database.

The performing of the step (1) before the bending process makes it possible to simulate the bending process for any of materials produced by different makers, under different specifications, in different batches and with different thicknesses by testing the material. If the range of kinds, specifications and batches of materials stored in the database is fully comprehensive, the relationship between the forming angles and the amounts of press of a corresponding material can be obtained directly from the database before the bending process, and high-speed accurate control can be achieved. If the database is incomprehensive, there is likelihood that a material similar to a metal sheet to be process cannot be found in the database. In that case, before the first bending, additional time needs to be spent in simulating the bending process of the metal sheet to be processed in order to obtain the relationship between the forming angles and the amounts of press.

Referring to the drawings, detailed description will be hereinbelow provided for the present invention, citing a particularly preferable example for the purpose of making the foregoing contents of the present invention more easily understood.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing how a database of the present invention is established;

FIG. 2 is a flow chart showing how a blanking process and a bending process of the present invention are performed;

FIG. 3 shows flow stress curves of generally-used cold-rolled steel sheets;

FIG. 4 shows a finite element simulation model for forming;

FIG. 5 is a schematic diagram showing how the forming is simulated;

FIG. 6 shows comparisons of simulated amounts of springback with experimental amounts of springback;

FIGS. 7A to 7D are schematic diagrams each showing a relationship between an amount of press of an upper die and a bending angle during a bending process;

FIG. 8 shows a relationship between bending angles (approximately 86 degrees to 96 degrees) and amounts of press of the upper die which is obtained when a bending process is simulated;

FIG. 9 shows a relationship between the bending angles (approximately 86 degrees to 96 degrees) and amounts of springback which is obtained when the bending process is simulated;

FIG. 10 shows a relationship between forming angles (approximately 86 degrees to 98 degrees) and the amounts of press of the upper die for each of materials A, B;

FIG. 11 shows a relationship between forming angles (approximately 86 degrees to 98 degrees) and amounts of press of the upper die for a material X; and

FIG. 12 is a schematic diagram of a bending die.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments for carrying out the present invention will be hereinafter described in detail with reference to the accompanying drawings.

Establishing a Database

The present invention provides a method for controlling bending of a metal sheet comprising the steps of:

(1) establishing a relational database on a relationship among material parameters, forming angles, amounts of springback, and amounts of press of an upper die,

the establishing including

-   -   establishing a finite element simulation model for forming based         on a structure parameter of a die used for a bending process,         obtaining a simulated amount of springback corresponding to a         forming angle by inputting a material parameter of a metal sheet         for a simulation into the finite element simulation model for         the forming and thus simulating the bending process of the metal         sheet for the simulation, and using the simulated amount of         springback as an amount of springback to be added to the         relational database, and     -   after measuring material parameters of different metal sheets,         simulating the bending process of each different metal sheet         based on the obtained finite element simulation model for the         forming, and for each of the different metal sheets,         establishing a relational database on a relationship among a         material parameter, a forming angle, an amount of springback,         and an amount of press of the upper die;

(2) blanking and reverse-engineering a material parameter of a metal sheet to be processed,

the blanking and reverse-engineering including

-   -   blanking the metal sheet to be processed, and thereby obtaining         a blank, and     -   obtaining an experimental load-stroke curve of the metal sheet         to be processed during a blanking experiment by measurement,         establishing a finite element simulation model using a method         for reverse-engineering an actual physical parameter of the         metal sheet, simulating the blanking, and repeatedly         reverse-engineering the material parameter of the metal sheet to         be processed,

(3) comparing the material parameters in the relational database with the material parameter of the metal sheet to be processed which is obtained by the reverse-engineering,

-   -   if a reference material parameter, between which and the         material parameter of the metal sheet to be processed an error         is less than a second threshold value, exists in the relational         database, performing a bending operation, and     -   if a reference material parameter, between which and the         material parameter of the metal sheet to be processed an error         is less than a second threshold value, does not exist in the         relational database, like in the establishing the relational         database, simulating the bending process using the material         parameter of the metal sheet to be processed which is obtained         by the reverse-engineering, adding a relationship which is         obtained by the simulation among the material parameter of the         metal sheet to be processed, a forming angle, an amount of         springback, and an amount of press of the upper die to the         relational database, making the material parameter of the metal         sheet to be processed which is obtained by the         reverse-engineering become the reference material parameter of         the metal sheet to be processed in the relational database, and         subsequently performing the bending operation; and

(4) performing the bending operation,

the performing including

-   -   inputting a bending angle, and     -   calculating a needed amount of press of the upper die based on a         formula representing a relation between the forming angle         corresponding to the reference material parameter and the amount         of press of the upper die of the die of a bending machine for         performing the bending operation, sending the needed amount of         press of the upper die to the bending machine, and controlling         the bending machine in a way that the bending machine bends the         blank.

The structure parameter of the die used for the bending process includes a draw radius of the upper die and an opening of a lower die in the die.

In order to establish the finite element simulation model for the forming, an actual amount of springback of the metal sheet for the simulation is obtained by performing a bending test, and a difference between the actual amount of springback and the simulated amount of springback is calculated, and if the difference is greater than a first threshold value, the simulation is repeatedly performed by refining a grid size of the finite element simulation model for the forming until the difference becomes less than the first threshold value.

Each material parameter used to establish the relational database on the relationship among the material parameters, the forming angles, the amounts of springback, and the amounts of press of an upper die includes a performance parameter and a flow stress curve of a material measured by a tensile test for detecting the material.

The bending process is simulated, and a relationship between a different bending angle θ′ and an amount ΔH of press is recorded, subsequently, an amount Δθ of springback corresponding to the different bending angle θ is predicted using the finite element simulation model, and a relationship between the different bending angle θ′ and the amount Δθ of springback is obtained, and based on θ=θ+Δθ representing a relationship among a forming angle θ, the bending angle θ′ and the amount Δθ of springback, a relationship between the forming angle θ and the amount Δθ of press is obtained

If the reference material parameter, between which and the material parameter of the metal sheet to be processed an error is less than the second threshold value, does not exist in the relational database, the bending process is simulated by inputting the material parameter of the metal sheet to be processed, which is obtained by the reverse-engineering, into the finite element simulation model, thereby, a relationship between a forming angle θ of the metal sheet to be processed and an amount ΔH of press is obtained, and the relationship between the forming angle θ of the metal sheet to be processed and the amount ΔH of press is sent to the relational database.

Descriptions will be hereinbelow provided for an example of the present invention while referring to specific numerical values. Subjects of an examination for the example are two low-carbon steel sheets each with a sheet thickness of 1.2 mm, which are similar but are produced by the respective makers. The steel sheets are respectively denoted by reference signs A, B. A tension test is performed on the two materials A, B using a Shimadzu's universal tensile testing machine at room temperature. Thereby, a load displacement curve is obtained from each of the materials A, B. Furthermore, for each material, a flow stress curve and a mechanical performance parameter are calculated based on a theoretical formula, and are used as input parameters to evaluate the mechanical characteristic of the material in simulating the bending process. FIG. 3 shows the flow stress curves obtained from the respective materials. It will be explained that albeit similar, the materials produced by the respective different makers are very different from each other in terms of the flow stress curve, and in terms of the mechanical performance. It will be shown how much the materials are different from each other in terms of the performance.

One may consider that the right-angle bending process is the most generally-used process in the actual industrial production. For this reason, the example will be explained citing the right-angle bending process. Parameters such as the draw radius of the upper die and the opening of the lower die in a needed bending die differ according to the thickness of an object to be bent. When a thinner sheet is the object to be bent, a smaller draw radius is selected for the upper die, and a smaller opening is selected for the lower die. When a thicker sheet is the object to be bent, a larger draw radius is selected for the upper die, and a larger opening is selected for the lower die. Because the example will be explained citing a case where the thickness of a sheet material to be bent is 1.2 mm, generally-used parameters suitable to bend the sheet material with the thickness of 1.2 mm are selected for the die. As shown in FIG. 12, the included angle of the upper die is 86 degrees, the draw radius of the upper die is 1.2 mm, and the opening of the low die is 8 mm. As shown in FIG. 4, a simplified finite element simulation model for the bending using finite element simulation software LS-DYNA is established based on the parameters of the actual bending die, that is to say, 86 degrees as the included angle of the upper die, 1.2 mm as the draw radius of the upper die, and 8 mm as the opening of the lower die. In the finite element simulation model, the die is defined as a rigid body, that is to say, as a body having no possibility of deforming, and the flow stress curve measured in the tension test is used as an input data for the finite element simulation model. A finite element simulation is performed on the process of the bending of the sheet material, and thereby the sheet material bent at 90 degrees is simulated as shown in FIG. 5. Thereafter, an amount of springback is predicted by simulating a simulated amount of springback corresponding to the bending angle by reuse of the finite element method. The example uses the case of the right-angle bending process in which: the thickness of the sheet material to be bent is 1.2 mm; the included angle of the upper die is 86 degrees; the draw radius of the upper die is 1.2 mm; and the opening of the lower die is 8 mm. These specific numerical values are used only for the purpose of explaining the example. The present invention is not limited to the specific numerical values, and uses arbitrary numerical values depending on the actual necessity.

In order to examine the accuracy of the thus-established finite element simulation model, the present invention calculates actual amounts Δθ of springback of the materials A, B by: bending the materials A, B using the die shown in FIG. 12 according to the actual production conditions; measuring the angles θ of bend of the materials A, B after bending before their springback; removing the upper die, and measuring the angles θ of bend of the materials A, B after their springback; and subtracting the angles θ of bend of the materials A, B after their springback from the angles 0′ of bend of the materials A, B before their springback. If comparison of the actual amounts of springback with the simulated amounts of springback shows that an error between the actual and simulated amounts of springback is less than a first threshold value, for example 2%, the accuracy of the established finite element simulation model for the bending can be considered as reliable. If the error between both amounts is greater than 2%, the grid size of the finite element simulation model is reasonably refined until the error between the simulated and experimental amounts of springback becomes less than 2%. As for the refinement of the finite element simulation model in the example, the finite element grid size in a direction of the thickness of the sheet material is the most influential in the simulation accuracy. Accordingly, as the grid in the direction of the thickness becomes thinner, the accuracy thereof becomes higher. Meanwhile, improvement in the accuracy by continually refining the grid size is limited when the grid in the direction of the thickness of the sheet material is refined to become equal to one seventh of the thickness of the sheet material. However, the simulation time becomes very long if the grid becomes too thinner. In this context, in the example, the optimal grid size in the direction thickness is one seventh of the thickness of the sheet material. FIG. 6 shows comparison of the simulated result and the experimental result for each of the materials A, B in the example. The error in the simulated amount of springback is 1% for the material A, and 1.5% for the material B. The simulation accuracy satisfies the requirement that the error should be less than 2%. For this reason, this finite element simulation model for the bending will be used as a universal model for the following simulation prediction of other materials. In the present invention, the first threshold value is 2%. However, the first threshold value is not limited to this specific numerical value, and may be arbitrarily changed depending on the actual necessity.

In the bending process, a relationship exists between the bending angle and the amount of press of the upper die. For this reason, as shown in FIGS. 7A to 7D, the angle amount of bend of a metal sheet can be controlled by controlling the amount of press of the upper die of a servo bending machine. The relation between different bending angles and the amounts of press of the upper die can be obtained by: simulating the entirety of the bending process performed on the material A based on the above-established finite element simulation model for the bending; and recording the various amounts of press of the upper die and the corresponding bending angles. Since the target bent angle for the example is 90 degrees, a focus is placed on the state of being bent at approximately 90 degrees. Thus, FIG. 8 shows an obtained relationship between bending angles (approximately 86 degrees to 96 degrees) and amounts of press of the upper die, as well as amounts of springback according to angle bending. Thereby, a relationship between different bending angles (approximately 86 degrees to 96 degrees) and amounts of springback is obtained, and is shown in FIG. 9.

A relationship between forming angles and the amounts of press of the upper die can be obtained based on a relationship among the forming angles θ, the bending angles θ′ and the amounts Δθ of springback, that is to say, θ=θ′+Δθ. Based on a linear analysis, the relationship between the forming angles θ of the material A and the amounts ΔH of press of the upper die is obtained as θ=−36.719×ΔH+169.02. In FIG. 10, the relationship for the material A is represented by a line A. Similarly, the finite element simulation for the bending is performed on the material B, and the relationship between the forming angles θ of the material B and the amounts ΔH of press of the upper die is thereby obtained as θ=−36.656×ΔH+170.03. In FIG. 10, the relationship for the material B is represented by a line B.

If materials produced by different makers, materials under different specification, and materials in different batches are tested according to this method, a relational database can be established among performances of the materials, forming angles, amounts of springback and amounts of press of the upper die. For each of the materials different in thickness, a finite element simulation model can be established for the forming using the draw radius of the upper die and the opening of the lower die which correspond to the thickness of the object material to be bent. In addition, the relational database can be established among the performances of the materials, the forming angles, the amounts of springback and the amounts of press of the upper die . In this manner, the database can cover problems with the bending of the materials produced by different makers, under different specifications, in different batches, and with different thicknesses.

Blanking

Each time a metal thin sheet X is blanked, force which the sheet material receives from the blanking die during the blanking process and displacement of a blanked part are measured and recorded. Thereby, a load-stroke curve corresponding to the actual blanking of the sheet material and the pre-bent blank are obtained. A finite element simulation model for the blanking process is established using a method described in a prior patent application (its application number is 201310680718.5), and a parameter representing a mechanical performance of a material is set for the sheet material to be blanked. Thereby, a finite element simulation is performed on the blanking process. The analysis is carried out by: combining the established finite element simulations for the blanking based on the load-stroke curve corresponding to the actual blanking; and for each process, repeatedly reverse-engineering the mechanical performance parameter of the sheet material X. The result of the repetition of approximately 30 processes shows that: simulated values representing the load-stroke curve basically coincide with the load-stroke curve corresponding to the actual blanking; and an error between the two curves is less than 1%. Thereafter, the mechanical performance of the material obtained by the reverse-engineering is outputted, and the corresponding flow stress curve is drawn. For the detailed method for reverse-engineering the material performance, see the prior application whose application number is 201310680718.5.

Bending Example 1

The performance parameter of a material C obtained by the reverse-engineering is sent to a control unit of the bending machine. The control unit draws a flow stress curve based on the mechanical parameter of the material to be processed, and compares the thus-drawn flow stress curve with the flow stress curves of the materials stored in the database. If the flow stress curve of the material A basically agrees with the flow stress curve of the material C, and the error between the two curves is less than a second threshold value, for example 1%, the control unit understands the material A in the database is similar to the material C and selects the material parameter of the material A as a reference material parameter of the material C. Since the bending performance of a material depends on the mechanical performance of the material, the bending performance of the material C should be similar to the bending performance of the material A. Although the present invention sets the second threshold value at 1%, the second threshold value is not limited to this specific numerical value. The second threshold value may be arbitrarily changed depending on the actual necessity.

Thereby, the conditions for processing the material C can be determined using the relationship between the forming angle and the amount of press of the upper die obtained from the material A, that is to say, θ=−36.719×ΔH+169.02. The forming angle requirement is inputted into the control unit. In this example, the forming angle is 90 degrees. By substituting the angle in the relational formula, the needed amount ΔH of press of the upper die can be calculated as ΔH=(169.02−90)/36.719=2.15 [mm].

Subsequently, an instruction that the amount of press of the upper die should be set at 2.15 [mm] is sent to the bending machine. The bending machine bends the blank of the material C which has already been blanked, and measures the forming angle of the material C after the springback. If the forming angle of the material C after the springback measures 90.04 degrees, the forming accuracy is far higher than the industrial requirement of the process accuracy for formed products, that is to say, 90+0.5 degrees.

In a bending example 2, a case where no material to meet the error requirements is found in the database is referred to.

Bending Example 2

The performance parameter of the material X obtained by the reverse-engineering is sent to the control unit of the bending machine. The control unit draws a flow stress curve based on the mechanical parameter of the material to be processed, and compares the thus-drawn flow stress curve with the flow stress curves of the materials stored in the database. If the control unit finds no material to meet the error requirements in the database, that is to say, if a reference material parameter, between which and the material parameter of the metal sheet X to be processed an error is less than the second threshold value, does not exist in the relational database, the control unit understands that the performances of the materials stored in the relational database is very different from the performance of the material X to be processed. In this case, the bending process to be performed on the material X to be processed needs to be simulated by reuse of the finite element simulation model for the bending.

A flow stress curve of the material X to be processed obtained by reverse-engineering is sent to the finite element simulation model for the bending, and a bending is thus simulated. A relationship between forming angles of the material X to be processed and amounts of press of the upper die is obtained. FIG. 11 shows the thus-obtained relationship. In addition, a relationship among the performance of the material to be processed, the corresponding forming angle and the corresponding amount of press of the upper die is sent to the database, and the contents of the relational database is thus complemented. In a case where the flow stress curve of a metal sheet M to be process basically agrees with the flow stress curve of the material X, the material parameter of the metal sheet X to be process obtained by the reverse-engineering becomes the reference material parameter of the metal sheet M to be processed which is stored in the relational database. After that, the control unit determines appropriate processing conditions based on the relationship between the forming angle corresponding to the material X to be processed and the amount of press of the upper die which is stored in the relational database. Moreover, when bending the sheet material X, the control unit can find the material X directly from the relational database to determine the processing conditions based on the relationship between the forming angle and the amount of press of the upper die which both correspond to the material X. The method refers to the forming example 1.

With this taken into consideration, as the number of materials stored in the relational database becomes greater, a material whose performance is in extremely high agreement with the performance of the material to be processed can be more easily found from the relational database. This makes the relationship between the forming angles of the materials and the amounts of press of the upper die in the relational database become better matched to the materials to be processed, and increases the accuracy of the bent products. This also makes no finite element simulation necessary to be performed for the forming, and thus increases the control speed. Even if a material whose performance is in agreement with the performance of the material to be processed cannot be found from the material database, the control system can complement the relational database by performing a simulation on the bending using the material parameter obtained by reverse-engineering when performing the blanking process; and predicting the relationship between the forming angle of the material to be processed and the amount of press of the upper die. Next time a similar material is bent, the control system uses data directly from the relational database, and accordingly need not perform a simulation on the bending.

The forgoing example is used only for the purpose of explaining the principle and effect of the present invention using the instance, and does not limit the present invention. Those skilled in the art can modify or change the foregoing example without departing from the spirit or scope of the present invention. For this reason, all the equivalent modifications and changes which those skilled in the art complete without departing from the spirit or technical concept disclosed in the present invention shall be included in the claims of the present invention. 

What is claimed is:
 1. A method for controlling bending of a metal sheet comprising the steps of: (1) establishing a relational database on a relationship among material parameters, forming angles, amounts of springback, and amounts of press of an upper die, the establishing including establishing a finite element simulation model for forming based on a structure parameter of a die used for a bending process, obtaining a simulated amount of springback corresponding to a forming angle by inputting a material parameter of a metal sheet for a simulation into the finite element simulation model for the forming and thus simulating the bending process of the metal sheet for the simulation, and using the simulated amount of springback as an amount of springback to be added to the relational database, and after measuring material parameters of different metal sheets, simulating the bending process of each different metal sheet based on the obtained finite element simulation model for the forming, and for each of the different metal sheets, establishing a relational database on a relationship among a material parameter, a forming angle, an amount of springback, and an amount of press of the upper die; (2) blanking and reverse-engineering a material parameter of a metal sheet to be processed, the blanking and reverse-engineering including blanking the metal sheet to be processed, and thereby obtaining a blank, and obtaining an experimental load-stroke curve of the metal sheet to be processed during a blanking experiment by measurement, establishing a finite element simulation model using a method for reverse-engineering an actual physical parameter of the metal sheet, simulating the blanking, and repeatedly reverse-engineering the material parameter of the metal sheet to be processed, (3) comparing the material parameters in the relational database with the material parameter of the metal sheet to be processed which is obtained by the reverse-engineering, if a reference material parameter, between which and the material parameter of the metal sheet to be processed an error is less than a second threshold value, exists in the relational database, performing a bending operation, and if a reference material parameter, between which and the material parameter of the metal sheet to be processed an error is less than a second threshold value, does not exist in the relational database, like in the establishing the relational database, simulating the bending process using the material parameter of the metal sheet to be processed which is obtained by the reverse-engineering, adding a relationship which is obtained by the simulation among the material parameter of the metal sheet to be processed, a forming angle, an amount of springback, and an amount of press of the upper die to the relational database, making the material parameter of the metal sheet to be processed which is obtained by the reverse-engineering become the reference material parameter of the metal sheet to be processed in the relational database, and subsequently performing the bending operation; and (4) performing the bending operation, the performing including inputting a bending angle, and calculating a needed amount of press of the upper die based on a formula representing a relation between the forming angle corresponding to the reference material parameter and the amount of press of the upper die of the die of a bending machine for performing the bending operation, sending the needed amount of press of the upper die to the bending machine, and controlling the bending machine in a way that the bending machine bends the blank.
 2. The method for controlling bending of a metal sheet according to claim 1, wherein the structure parameter of the die used for the bending process includes a draw radius of the upper die and an opening of a lower die in the die.
 3. The method for controlling bending of a metal sheet according to claim 1, wherein in order to establish the finite element simulation model for the forming, an actual amount of springback of the metal sheet for the simulation is obtained by performing a bending test, and a difference between the actual amount of springback and the simulated amount of springback is calculated, and if the difference is greater than a first threshold value, the simulation is repeatedly performed by refining a grid size of the finite element simulation model for the forming until the difference becomes less than the first threshold value.
 4. The method for controlling bending of a metal sheet according to claim 1, wherein each material parameter used to establish the relational database on the relationship among the material parameters, the forming angles, the amounts of springback, and the amounts of press of an upper die includes a performance parameter and a flow stress curve of a material measured by a tensile test for detecting the material.
 5. The method for controlling bending of a metal sheet according to claim 1, wherein the first threshold value is 2%.
 6. The method for controlling bending of a metal sheet according to claim 1, wherein the bending process is simulated, and a relationship between a different bending angle θ′ and an amount ΔH of press is recorded, subsequently, an amount Δθ of springback corresponding to the different bending angle θ′ is predicted using the finite element simulation model, and a relationship between the different bending angle θ′ and the amount Δθ of springback is obtained, and based on θ=θ′+Δθ representing a relationship among a forming angle θ, the bending angle θ′ and the amount Δθ of springback, a relationship between the forming angle θ and the amount ΔH of press is obtained
 7. The method for controlling bending of a metal sheet according to claim 1, wherein the second threshold value is 1%.
 8. The method for controlling bending of a metal sheet according to claim 7, wherein if the reference material parameter, between which and the material parameter of the metal sheet to be processed an error is less than the second threshold value, does not exist in the relational database, the bending process is simulated by inputting the material parameter of the metal sheet to be processed, which is obtained by the reverse-engineering, into the finite element simulation model, thereby, a relationship between a forming angle θ of the metal sheet to be processed and an amount ΔH of press is obtained, and the relationship between the forming angle θ of the metal sheet to be processed and the amount ΔH of press is sent to the relational database. 